PITTSBURGH (KDKA) — A large oil sheen formed on the Monongahela River Tuesday afternoon and river traffic was halted when a tugboat transporting several barges became stuck on the water.

It all happened near the area of the Rankin Bridge and the Homestead Grays Bridge.

A second tugboat was brought in to help move the barges, which became wedged sideways, blocking the all movement on the river in that area.

Meanwhile, authorities say the oil sheen was not caused by a spill, but was most likely dredged up from the bottom of the river because the stuck tugboat’s engine was being worked hard to move the boats.

The tugboat, called the Carrie Mays, was traveling downstream on the Mon River when witnesses say it appeared the empty barges in its load may have it a shallow area of the river.

“It’s my understanding it’s real shallow on that side, so he probably ran aground,” said Judi Jacquin, a witness. “So, that’s when he came over here and tied himself in and he started running the engines hard.”

Passersby could tell right away that the tugboat was in trouble.

“He had the front end, after about an hour of revving his engines, looked like he was trying to put all his force to turn the barges to the left,” said Ed Urban, another witness. “He actually had the front end down about 200 feet to the left. All of the sudden, the wind blew and just pushed him right back to where he started from.

“Initially, we felt he was just trying to keep himself from drifting further across,” added Jacquin. “And then you could see him begin to work and work and work and you could see the front end begin to slowly move back towards the river.”

Early reports of the sheen on the river pointed to a spill of some sort, but closer investigation found no leaks and no danger from the incident.

“From the best we could tell, he does not have an active leak of any type of diesel fuel,” said Chief Alvin Henderson, of Allegheny County Emergency Management. “Also, there was a concern obviously with the one barge with hazardous materials. That barge looks like it is intact, not leaking any type of chemicals at this time.”